The invention concerns generally the technology of setting up and maintaining wireless connections to terminals that are collectively moving on board a carrier vehicle. Especially the invention concerns the technology of providing WLAN connections to trains and other railway vehicles.
WLAN or Wireless Local Area Network is a general designation for all technologies that enable computers or computer-like terminals to exchange digital data wirelessly with multiple other similar devices nearby. At the original date of writing this description the data rates in WLAN connections are of the order of 11 Mbit/s. According to an established presumption, a WLAN must also have a connection to an external, widely spread public network, preferably the Internet, so that a terminal of a WLAN has practically unlimited access to all over the world.
A WLAN connection is believed to become an essential part of everyday life in a very wide scope of environments. An ongoing developmental trend is the increasing appearance of so-called WLAN hotspots at locations where a number of people may have the need and opportunity to use their terminals for accessing a network simultaneously. An interesting special case of such locations consists of public transport vehicles, such as buses, trains and aeroplanes. Since the range between a terminal and a base station in a typical WLAN connection is of the order of the dimensions of e.g. a railway carriage, an obvious suggestion for equipping a train with WLAN connections is to install a base station within the carriage for setting up the actual WLAN and to arrange for a separate centralised radio link between that base station and the stationary outside world.
FIG. 1 illustrates a basic case where a general-purpose cellular radio system comprises a number of fixed base stations 101 to 110, each having a coverage area or cell so that schematically the cells cover a certain geographical area as a hexagonal grid. A railway track 111 crosses through said area, and thus cuts through a number of said cells. At one location the railway track 111 passes through a tunnel 112. The most straightforward alternative for setting up a radio link between a WLAN base station 113 in a railway carriage and the stationary outside world would be to equip the WLAN base station 113 with a terminal transceiver of the general-purpose cellular radio system, and to treat the moving WLAN as if it was a single moving terminal. Numerous drawbacks make such a solution not feasible. Among these are the typically insufficient bandwidth available for terminals in the general-purpose cellular radio system, uncertainty of coverage in difficult locations like the tunnel 112, and unguaranteed capacity in situations where one of the cells may already have an excessive number of ordinary terminals with active communication connections.
FIG. 2 illustrates a more advanced solution in which one takes advantage of the fact that unlike the terminals of a general-purpose cellular radio system, a train will not appear in arbitrary geographical locations, but will only move along a very well defined path. In FIG. 2 the path of the railway track 111 is covered by a number of dedicated cells 201 to 209, including a specific cell 210 in the tunnel 112 that utilises e.g. the known so-called leaking cable technique. In this solution at least the capacity issues can be solved more easily than in the solution of FIG. 1, because there are no other terminals competing with the moving WLAN base station 113 that appears as a moving terminal in the system consisting of the cells 201 to 210. However, there remain important questions to be solved. In order to cover significant portions of a railway network with dedicated cells, there are easily needed several hundreds or thousands of fixed base stations. How could these be connected to a core network operative on the background in a cost-effective way, and how to effectively manage the mobility of the end users' terminals?
Additionally there are serious questions related to Doppler shift and fading. A digital radio transmission of the kind used in the connections between the moving WLAN and the fixed base stations involves transmitting packets, each of which comprises a so-called training sequence. A receiver uses the known form of the training sequence to calculate a channel estimate, which acts as control information to a matched filter that tries to counteract the detrimental effects of nonlinearities in the radio channel. Updated channel estimates are only available as frequently as new packets come in. If packets are very long, the time interval between the calculation of consecutive channel estimates becomes long, which increases the risk that the actual impulse response of the channel changes too much from the most recently calculated channel estimate, causing a packet to be lost. On the other hand, if packets are very short they enable frequent updating of the channel estimate, but the relative amount of transmission capacity available for payload data gets smaller.
Advantages may be gained from the present invention by providing solutions for setting up and maintaining WLAN connections within predictably moving vehicles.
Another way of gaining advantages from the invention is by providing a cost-effective way of connecting dedicated, fixed base stations with a core network as a part of such a solution. Another way of gaining advantages from the invention is by providing a method and appropriate hardware for managing the mobility of end users' terminals that move within a predictably moving vehicle. Another way of gaining advantages from the invention is by providing radio interface mechanisms between fixed base stations of said kind and a WLAN part moving within a predictably moving vehicle.
One way of gaining advantages from the invention is by building an infrastructure comprising a chain of dedicated access points, by properly linking the access points to each other and to inner network structures, and by setting up a mobility management scheme that takes advantage of predictability of the movement of a vehicle.